Design of a narrow-bandwidth refractive index sensor based on a cascaded few-mode long-period fiber grating.

This work presents a design of a cascaded few-mode long-period fiber grating (FM-CLPFG) refractive index sensor with a narrow bandwidth that is based on the feature of narrow-bandwidth loss peak of few-mode long-period fiber grating (FM-LPFG) and that further reduces the loss-peak bandwidth of a single FM-LPFG by cascading. On the basis of the coupled-mode theory of FM-LPFG, the mutual interference between the loss peaks of each mode is reduced, and the loss-peak coupling intensity is ensured by selecting the appropriate grating period and grating length. Furthermore, the influence of the cascaded fiber length and the number of cascaded grating segments on the loss-peak bandwidth are analyzed. Based on the above designed parameters, the FM-CLPFG narrow-bandwidth sensor with a bandwidth of about 1 nm is designed. The results show that the sensitivity of this sensor is available to 2410 nm/RIU, with the surrounding refractive index changing from 1.4445 to 1.4475.